In general, the invention involves screening methods for identifying modulators of metabolism of any of a wide range of foreign compounds, collectively termed xenobiotics.
A number of cytochrome P450 (CYP) enzymes able to metabolize diverse substrates serve as a primary defense against potentially deleterious effects of xenobiotic compounds. Induction of the expression of individual CYP genes in response to particular xenobiotics is a central component of this metabolic mechanism. One of the best characterized of these responses is the induction of specific CYP genes by a diverse group of agents known as “phenobarbital-like” inducers. Exposure of animals to any of a chemically diverse series of compounds exemplified by phenobarbital (PB) results in a potent activation of expression of a specific subset of CYP enzymes and other proteins associated with xenobiotic metabolism. In the mouse, these PB-like inducers increase expression of CYP2B10 and several other genes. The pesticide contaminant 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, referred to as TCPOBOP, is generally considered to be the most potent of this group of inducers.
Xenobiotics, such as therapeutic drugs, insecticides, polycyclic hydrocarbons, and some natural products, are often metabolized via oxidation reactions catalyzed by CYP enzymes. These reactions add hydrophilic groups to xenobiotics, allowing the body to rid itself of these noxious or simply insoluble materials. For example, oxidation of polycyclic aromatics produces epoxides, which are very reactive electrophilic groups. Usually these epoxides are rapidly hydrolyzed into hydroxyl groups which are then coupled to other groups, producing compounds water-soluble enough to be excreted. Unfortunately, the intermediate epoxides may also be released into the cell as highly reactive electrophiles, possibly reacting with negatively charged groups in DNA and causing changes in the DNA sequence. Reactive oxygen species generated during metabolism of cocaine by CYP enzymes in humans has been associated with mutagenesis and chromosome breakage.
CYP-mediated metabolism may also result in other undesired effects, such as the rapid degradation of a therapeutically active compound, lowering its half-life in vivo. Alternatively, CYP enzymes may convert a prodrug into an active drug at a faster than desired rate resulting in a toxic concentration of the active drug in vivo. Additionally, the activation of CYP enzymes by the administration of a therapeutically active compound or exposure to another foreign compound may result in faster metabolism of a second therapeutically active compound, reducing its effectiveness or increasing its toxicity.
Because of the potentially deleterious effects of compounds that activate CYP enzymes, improved methods are needed to determine which compounds activate CYP-mediated metabolism and, thus, might cause side-effects if administered to humans. These compounds may thereby be eliminated from drug development or chemically modified to generate related compounds with less ability to activate CYP enzymes.